1. Field of the Invention
This invention relates to infrared detectors cooled to low temperature in a cryostat, and, more particularly, to minimizing thermal fluctuations of the detector.
2. Description of the Prior Art
In recent years, many devices have been developed which detect electromagnetic radiation in many areas of the spectrum. Particular attention has been given to the development of detectors which provide an output signal when radiation in the infrared portion of the spectrum is received. Characteristically, these detectors function most efficiently when they are cooled to cryogenic temperature, for example, 77.degree. K. Rapid and efficient cooling of the detector is, of course, desirable. Such infrared detectors are used in a diverse variety of application, such as a guidance sensor on a missile which is intended to seek and destroy IR-emitting aircraft, rockets and the like, in night viewing devices, terrain scanning devices as, for example, from a synchronous satellite, and other applications. In any event, the detectors characteristically operate more efficiently when cooled to cryogenic temperatures. One mode of cooling the detectors to low temperature levels is to provide a source of cryogenic fluid such as argon or nitrogen under comparatively high pressure and bleed the cryogenic fluid into an expansion area adjacent the detector heat load, the expansion thereof inducing cooling thereof by virtue of the Joule-Thomson effect.
Over the years, the sensitivity of various infrared radiometers has improved and the use of mercury cadmium telluride detectors has increased. These detectors require, in many cases, an extremely stable detector temperature to function properly. However, when cooling infrared detectors, Joule-Thomson cryostats tend to produce a temperature fluctuation at the detector. Thermal fluctuations are particularly observed in throttling cryostats, which turn on and off as necessary to meet demand for cryogenic fluid. Most important to a number of applications is the rate of temperature change of the detector. Devices which have a relatively low frequency response will interpret detector temperature changes at rates higher than a certain threshold (which is determined by the low frequency response) as infrared radiation signals.
Attempts to reduce the rate of temperature change produced by typical Joule-Thomson cryostats (or other cooling devices such as closed cycle coolers) have involved thermophonic dampers which utilize only an increased mass, resulting in an increased thermal inertia. Such dampers, however, require an undesirable increased cool down time. Thermophonic dampers which utilize only an increased thermal resistance have also been employed. However, such dampers have increased detector temperature penalties associated therewith, including significantly reduced cool-down times and increased shock and vibration vulnerability to the package.